Recently, a portable phone or a mobile communication terminal is equipped with a compact digital camera or a digital video camera employing a solid state image sensor, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) image sensor. Such an image sensor has become scaled-down, so that an optical system used for the image sensor is needed to have a small size and high performance.
In addition, an optical system according to the related art includes first to fourth lenses, a filter, and a light receiving device. In this case, the first to fourth lenses are sequentially arranged upward from an object side. In addition, the first and third lenses may have positive refractive power, and the second and fourth lenses may have negative refractive power. In addition, the second lens may be designed so that the refractive power of the second lens is greater than that of other lenses.
The first lens may have a surface convex toward the object side, and the second lens may have a surface concave toward an image side. The filter may include an infrared filter, and the light receiving device may include a CCD image sensor or a CMOS image sensor.
The above small optical system is disclosed in Korean Patent Application No. 10-2007-0041825.